The screenplay was written by Waldemar Young, based on a story "The Mockingbird" by Tod Browning (which was originally supposed to be the film's title).

A title card introduces the setting as London's Limehouse district, "with its lust, greed and love, a sea of fog, a drama of human faces.” A cheap music hall is overseen by the tough Dan Tate, who also manages a small gang of thieves under the alias "The Blackbird."

As a cover, though, Tate also poses as his own deformed and noble twin brother, affectionately known as the "Bishop of Limehouse," who supposedly lives above an adjoining mission for the poor.

While the police hear two apparently different voices talking, Dan changes clothes and contorts his body, with his arms and legs at extreme angles, then making his way down the stairs with a crutch as The Bishop, verifying The Blackbird's alibi.

Later at the music hall, a "slumming" group of upper-class Londoners arrives, led by the apparently wealthy and respectable Bertram P. Glade, who is really a thief known to Dan and others as "West End Bertie."

As The Bishop, Dan works to turn Bertie and Fifi against each other so that he can declare his own love to her, but one of The Blackbird's men tells the police who actually shot the officer.

When Polly enters, Dan asks her to burn The Blackbird's clothing, which he had hidden in the corner, and she finally realizes that the two "brothers" are the same man.